In the video above, it showed the horse walking, troting and galloping. When the horse is acting out the walk, it is walking with he same pace and does not speed of slow down. This also goes for when it trot and galloped, the speed was constant. Even though all three of them differ in ways of walking, each of them had there own constant velocity.
Lastly, this video shows the concept of acceleration. If you do not want to watch the whole video, skip to the time 1:10. When Donald Duck began to chase the three little ducks he started out at a somewhat constant speed, but he eventually sped up and continued to do so until he caught up with them. This is called acceleration, where the velocity changes due to the change in speed and/by direction.